Television receiving system



2, 1942. A. H. ROSENTHAL 2,270,232

' TELEVISION RECEIVING SYSTEM Filed Jan. 12, 1939 i 3 Sheets-Sheet l J 26, 1942. A ROSENTHAL 2,270,232

TELEVISION RECEIVING SYSTEM Fil ed Jan. 12, 1939 3 Sheets-Sheet 2 Jan 2@, 1942- A. H. ROSENTHAL TELEVISION RECEIVING SYSTEM 5 Sheets- -Sheet 3 Filed Jan. 12, 1939 I I[ III N YYI NW1 1! KXIXIIXHIKN Patented Jan. 20, 1942 TELEVISION RECEIVING SYSTEM Adolph Henry Rosenthal, New York, N. Y.

Application January 12, 1939, Serial No. 250,649 In Great Britain January 18, 1938 17 Claims.

The present invention relates to cathode ray tube apparatus for television receivers employing a supersonic wave light modulating devices Television receivers employing such light modulating devices have been hitherto of the mechanical optical type, typical examples of such receivers being described in British Patent Speci- Iication No. 439,236. In these receivers a high frequency electrical oscillation modulated in amplitude in accordance with the received picture signals is applied to a vibrating member, such as a piezo-electric crystal, and this member generates a train of moving compressional waves in a liquid, the amplitude of the wave train at successive points along its length thus corresponding to the intensity of successive picture points. A beam of light is passed through the wave train and thereby suffers optical diffraction. By means of an optical stop the diffracted and non-diffracted portions of the light beam are separated from one another, and one of these parts is utilised by an optical system to form on a receiving screen an image of the moving wave train in the liquid, this image being in the form of a strip of light having a width equal to the width of a picture line and length determined by the length of the wave train. This image is moved in the line scanning direction by a high speed scanner, such as a mirror drum, in such a sense and at such a speed that the movement of the wave images is immobilized with respect to the screen, and in the frame scanning direction by a low speed scanning member, so that the complete picture is reconstituted on the screen. It is clear that in such a receiver a number of elemental areas of the receiving screen are simultaneously illuminated, this number being determined by the length of the Wave train imaged on the screen.

In present day cathode ray tube receiving apparatus, the area of the scanning spot, and hence the area of cross-section of the scanning beam, will determine the definition of the received picture, since only a single elemental area of the receiving screen is illuminated at a time. Hence the intensity of the reproduced picture can be increased only by increasing the current density of the beam if the definition of the picture is to remain unaltered. Severe limits to the possible amount of this increase exist, however. In the first place the fluorescent materials commonly employed as the receiving screen become saturated at high current densities so that beyond this point no increase of screen brightness can be obtained except by increasing the spot size and thus impairing the definition. Secondly, an increase in the current density of the beam shortens the life of the receiving screen, and at very high intensities this life becomes so short as to destroy the practicability of the device. These diiificulties become very acute in the case of the so-called projection type receivers, in which a very small and very bright fluorescent image is optically projected on an enlarged scale on to a receiving screen. Owing to the small sizeof the fluorescent image the spot size is very small and hence the current density must be very great.

It is an object of the present invention to provide a cathode ray tube television receiver in which these disadvantages are avoided.

According to the present invention there is provided television receiving apparatus comprisin a cathode ray tube provided with a photolelectric surface and an image screen, a supersonic wave light modulation device, optical means for forming on said surface an optical image of the moving wave train in said device and means associated'with said cathode ray tube for forming a corresponding electron image on said image screen and means for deflecting said image over said screen to immobilize the movement of the wave train represented therein with respect to said screen.

The image screen of the cathode ray tube may be the usual fluorescent or similar screen, or it may be a screen capable of secondary electron emission, in which case the emitted secondary electrons are focussed electron-optically on to a final fluorescent screen. Alternatively, it may be a screen, the transparency of which to ordinary or polarized light can be varied from point to point in accordance with the intensity of an electron beam falling upon it.

The photo-electric surface may act as the electron source for the tube, or its electron emission may be used to control the emission from an independent source, such as a thermionic cathode. In eithercase, by imaging the plane of the photo-electric surface on to the image screen with the aid of a suitable electron-optical system,

an electron image will be formed on the latter which corresponds to the optical image formed on the photo-electric surface.

Thus the total number of elemental areas of the image screen which are simultaneously excited by the electron image will be equal to the number of picture points represented in the wave train. This number will usually be -200 but, by suitable means to be described later, a

whole picture line can be simultaneously excited. Consequently the total beam current can be increased by a factor of at least 100 without increasing the current density beyond the limiting value referred to above. This results in not only a much brighter picture but also improves the tone gradation in the high lights since the image screen is no longer saturated. Alternatively the total beam current can be maintained at the lower value commonly in use to-day, in which case the current density becomes at the most one hundredth of its normal value. In this case the life of the image screen is greatly increased.

It will be noted that the cathode ray tube of the present invention has many features in common with the so-called image intensifiers, in which an optical image of a certain brightness is projected on to a photo-electric surface, is converted into a corresponding electron image on a fluorescent surface, where it is re-converted into an optical image of increased brightness, this increase being a function of the accelerating voltages applied to the eletrodes of the intensifier. In addition to this desirable property the apparatus of the present invention has the advantage over the normal image intensifier in that the picture points of the optical image on the photo-electric surface are in motion with respect to this surface. Consequently any non-uniformity in the sensitivity of this surface will equally affect all picture points and will therefore not appear in the final picture.

The invention will now be described by way of example with reference to the accompanying drawings: in which Fig. 1 shows schematically a preferred form of the invention;

Figs. 2-6 show schematically alternative forms of cathode ray tube for use in connection with Fig. 1;

Fig. 7 is an explanatory diagram;

Figs. 8 and 9 show schematically alternative forms of apparatus for carrying into effect the method described in connection with Fig. 7; and

Fig. 10 shows a further modification of the cathode ray tube.

' cathode ray tube is a supersonic wave light modulating device 7 of the type described in British Patent Specification No. 439,236, a light source 8 which illuminates the aperture 9, an opaque bar l0, a pair oflenses l l which form an image of the aperture 9 on the bar l0, and a lens l 2 which forms an image of the modulator 7 on the photoelectric surface 2. A piezo electric crystal which is energized by high frequency voltages modulated in amplitude in accordance with the re ceived picture signals from the source l3, sets up in the liquid M a travelling train of supersonic waves modulated in amplitude in accordance with the received picture signals. The lens l2 collects the light which is diffracted by the action of the waves and thus passes the bar it,

andutilizes it to form an image of the moving train of waves on the surface 2. This image will be that of a portion of a picture line, the length of the portion depending upon the length of the wave train in the liquid H, but the picture points reproduced in it are moving at a speed depending upon the velocity of the waves in the liquid. The'light incident upon the surface 2 gives rise to electrons which are focussed by the elements 311, 3b, 30, to form a corresponding electron image of a portion of a picture line on the image screen 6. The deflecting 'coils 5 move this image over the screen 6 in the line scanning direction (in the plane of the paper) in such a sense and at such a speed that the moving wave images represented in this electron image are immobilized with respect to the screen 6. The deflecting coils 4 move the image over the screen in the frame scanning direction, so that the complete picture is reconstr'ucted on the screen 6.

An alternative form of cathode ray tube is shown in Fig. 2. Here, the electrons from the surface 2 are imaged by the elements 3a, 3b, 30 on an image screen I5 consisting of a fine grid coated with secondary electron emitting material,

the image being swept over this screen by the deflecting coils, 4, 5, so that a complete electron image of the original picture is formed on this screen. Focussing elements I Get, I51), I60, image the secondary electrons emitted by the screen on to the fluorescent screen 6 where the visible image is created.

The effects of secondary electron amplification can be obtained by inserting secondary emitting grids in front of the deflecting coils as shown in Fig. 3, in which one such grid #1, maintained at a positive potential with respect to the surface 2, is shown. The coil 18 serves to focus the electrons from the surface 2 on to the grid 11, and the remaining elements form an electron image of the original picture on the fluorescent screen 6 from the secondary electrons emitted by the grid I1. If more than one grid is employed they must either be in close proximity or separated by electron imaging means such as the coil l8. Instead of both pairs of deflecting coils 4, 5 being between the grid I1 and the fluorescent screen 6, the line scanning coils 5 can lie between the surface 2 and the grid I1, and the frame scanning coils between the grid I1 and the screen 6. The secondary emitting surfaces can take any of the forms well known per se and used in the so-called electron multipliers. For example they may comprise a deposit of caesium on an oxide base, such as aluminium oxide.

Referring once more to Fig. 1, the width of the line image formed on the screen 6, i. e. its dimension at right angles to the plane of the paper, must be equal to the line width of the received picture. If the focussing system 3a, 3b, 3c is stigmatic, the ratio of the width to the length of this line image must be equal to the ratio of the width to the length of the optical line image formed on the surface 2. Consequently the width of the latter image is very small, and the total area of the photo electric surface which is illuminated is also very small. In certain cases this will necessitate a very high specific illumination of this surface, in order to obtain an adequate current in the electron beam, with possible harmful results to the surface. This difficulty can be overcome by arranging that the electron-optical system 3a, 3b, 3c is astigmatic, i. e. has a different optical power in the plane of the paper than in the plane at right angles thereto, and that the ratio of the width to length of the optical image on the photo-- electric surface is greater than the ratio of the width to length of the electron image formed of the screen 6, whereby a much greater area of the surface 2 can be utilized.

In the arrangements already described there is a possibility that the bright illumination of the screen 6 will reach the surface 2 and produce an optical back coupling. This can be avoided by arranging the surface 2 and the screen 6 in planes at an angle to one another, for example, a right angle. This is shown in Figs. 4 and 5, in which for sake of clarity, the deflecting coils 4, 5 and the electron optical system 3a, 3b, 3c are omitted. Light from the supersonic wave modulator is reflected from the mirror 20 on to the photo electric surface 2 to form an image of the supersonic waves train thereon, the length direction of this image being at right angles to the plane of the paper in Fig. 4, and in the plane of the paper in Fig. 5. The magnet coils 2i bend the electron beams proceeding from the surface 2 through a right angle so that they fall on the screen 6 at normal incidence, the focussing electron optical system (not shown) serving to focus these beams on to the screen 6 to form a corresponding electron image there, as in Fig. 1. This swept over the screen 6 by the line and frame deflecting coils (not shown), the line scanning direction being at right angles to the plane of the paper in Fig. 4 and in this plane in Fig. 5.

In addition to preventing the optical back-coupling between the surface 2 and the screen 6, this arrangement has the further advantage that the surface 2 need no longer be semi-transparent as is necessary in the arrangements of Figs. 1-3, but can be opaque. Such an opaque surface is much more sensitive than a semi-transparent surface.

Instead of using the magnet coils 2| to effect the bending of the electron beams, this can be achieved with the aid of an electron optical mirror, consisting of a suitable electron lens system to which is applied such potentials that a total reflection of the electron beams takes place.

This construction also enables an opaque fluorescent, screen to be used, as shown in Fig. 6, which illustrates a tube for the purpose of producing large size pictures by optical projection from a small intense image on a fluorescent screen. Here, the elements 2, 6, 20, and 2| perform the same function as in Fig. 4 to produce a small and very bright picture on the inside surface of the opaque fluorescent screen 6. The l ght from this image passes through the tube, and the projection lens 22 forms an enlarged image of the screen 6 on the viewing screen 23.

Instead of preventing the light from the image screen 6 from actually reaching the photo-electric surface 2 the undesired back coupling can be prevented by arranging that the photo-electric surface 2 is sensitive to light of a spectral range other than the spectral range of the light emitted by the screen 6. For example, a lithium photoelectric surface is particularly sensitive to ultraviolet light and is hardly affected by light emitted by the normal fluorescent screen. In this case the image of the wave train could be produced by using a mercury light source having the greater part of its emission in the neighbourhood of the ultra-violet mercury resonance line, and the necessary .lenses ll, [2 (Fig. 1) and at least the light entrance part of the tube would be constituted of quartz. The use of such a light source, or any mono-chromatic light source such as a. sodium lamp, for imaging the wave train on the surface 2, has the further advantage in that it enables the size of the source to be increased. This is due to the fact that no spreading of the diffraction spectra occurs in the plane of the bar I0 (Fig. 1), as when white light is used, so that substantially no overlapping of these spectra occurs even with a comparatively large source. Furthermore, if a source of light having its maximum emission in the red or infra-red portion of the spectrum is used, then the separation of the diffraction spectra becomes greater, again enabling a larger light source to be used.

From the foregoing it is obvious that the efficiency of the arrangement according to the invention increases with the number of picture elements which are simultaneously represented in the electron image on the image screen, the efficiency being the greatest when the elements of a complete picture line are thus represented. With the embodiments shown in Figs. 1-6 a difficulty arises if too many picture elements are thus represented, in that the brightness of the reproduced picture will not be uniform. This can be explained with reference to Fig. 7. The vertical bounding lines of the fluorescent screen are indicated at 30, and the vertical strips l-XIV indicate the elemental areas of one line of the picture to be reproduced on the screen the vertical dimension being much enlarged so that the various positions of the electron image during one line period can be shown one under the other, instead of being superposed, as in practice. This image includes eight picture elements and its initial position is shown at 3| and its final position at 32. After reaching the position 32 it will fly back to the position 3| again, and so on. It is seen that the area XIV will be illuminated for the duration time of one picture element, the area XIII for the duration time of two picture elements, and so on, up to the area VII, which will be illuminated for the duration time of eight picture elements. The areas IVI will also be illuminated for the duration time of eight picture elements. Consequently the brightness of each line will decrease uniformly over the portion VII-XIV, i. e. over a portion equal in length to the length of the electron image. Hence, if this effect is not to be noticeable, the length of the electron image must be comparatively short. If it is desired to use longer electron images, and in particular when it is desired to use an electron image-having the length of a picture line, special means must be provided to overcome this difiiculty. One method of achieving this is to form two optical images of the Wave train on the photo-electric surface 2, corresponding points in the two images being separated in the line scanning direction by a distance approximately equal to the equivalent length of a picture line. Electron images corresponding to these two optical images are then swept over the image screen, with the result that each elemental area is excited for the same length of time. This extra image is indicated at 33, and it will be seen that each element IXIV is now illuminated for the duration time of eight picture elements.

Methods of achieving this are illustrated in Figs. 8 and 9. In Fig. 8 two supersonic wave modulating devices 1a and 1b, situated so that corresponding points on them are separated by a distance approximately equal to the equivalent length of a picture line are imaged by a lens l2 two required optical images 34 of the wave trains are produced. The crystals of the devices 1a and lb are driven in parallel with the same modulated high frequency oscillation so that identical wave trains are produced in the two cells. In Fig. 9, the two optical images 34 are produced from a single modulating device I by the inclined mirrors 35 and the lens I2.

Instead of making the time of excitation substantially constant for all elemental areas of the image screen, the decreased time of excitation of the areas near the end of the line can be compensated for by a relative increase in the degree of excitation of these areas. This can be achieved by superimposing on the high frequency oscillation applied to the crystal of the modulating device (Fig. 1) a saw tooth voltage of a frequency equal to the line scanning frequency and adapted to increase the intensity of the supersonic waves towards the end of each line scanning period. Alternatively, asimilar modulation can be superimposed on the steady accelerating voltage applied to one of the electrodes 312 or 30 (Fig. 1) to increase the velocity of the electrons and hence the excitation of the image screen towards the end of the line scanning period.

It is preferable to use the synchronising signals during the fly-back period or an even longer period to decrease substantially the intensity of the electron image, or to deflect it out of the field of view, so that it produces no visible effect during this period. This can be achieved by reducing or making negative the accelerating voltage, by applying to the electrode 3b (Fig. 1-) a negative voltage derived from the synchronising signals.

If the length of the electron image approaches,

that of a complete picture line, this suppression may exist for a period which is much longer than the fly-back period, so that the screen 6 is only excited for a small portion of each picture line period. The line scanning deflection then only takes place during this short excitation period, and in consequence, its duration and extent are very much reduced. This enables a great simplification of the line scanning equipment to be achieved. Alternatively the electron image can be deflected out of the field of view by means of an extra voltage peak superimposed on the deflecting voltages applied to the coils (Fig. l), or by means of an extra deflecting voltage applied to a separate deflecting electrode or coil.

In-Fig. 10 is shown an alternative method of converting the optical image of the supersonic wave train into an electron image. The lens i2 forms an image of the supersonic wave light modulating device I on a grid 40 coated with photo-electric material. This grid is held at a positive potential with respect to a thermionic cathode 4i, and in front of it and close to it is a second grid 42, which is non-photo electric, and is held at a slight positive potential with respect to the grid 40. When the latter is illuminated with the optical image a space charge of electrons is formed between thegrids, this charge being difierent at diflerent points on the grids, in accordance with the intensity of illumination at these points. This spatial voltage distribution influences the electron beam from the cathode 4! to produce a spatial intensity distribution of the electrons, which when focussed electron optically on the image screen 6, gives rise to an electron image on the screen 6. The electron optical diaphragm 43 produces a divergent beam of electrons from the cathode 4! so that the whole of the space between the grids 40, 42 is traversed by electrons. Instead of two grids a single photo-electric grid composed of mutually insulated wires may be used, a suitable leakage path being provided so that the charges on the wires can leak away. Also the cathode system 4|, 43 can be replaced by a cathode having a large area. I

I claim as my invention:

1. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, means incorporated in said tube for forming an electron image of said train on said image 'screen, and deflecting means associated with said tube for deflecting said electron image over said screen to immobilize the picture points represented in said electron image with respect to said screen.

2. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, an electron-optical focussing system in said tube for focussing the electrons emitted by said surface to form an electron image of said train on said image screen, and deflecting means associated with said tube for deflecting said electron image over said screen to' immobilize the picture points represented in said electron image with respect to said screen. I

3. Television receiving apparatus comprising a cathode ray tube having a source of electrons, a photo-electric control grid and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said grid, an electron-optical focussing system in said tube for focussing the electrons passing said grid to form an electron image of said train on said image screen, and deflecting means associated with said tube for deflecting said electron image over said screen to mobilize the picture points represented in said electron image with respect to said screen.

4. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface, a secondary electron emitting image screen and a picture screen, a supersonic wave light maintaining device, a light source for illuminating said device, means for applying picture signals to said device to provide therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, means incorporated in said tube for forming an electron image of said train on said image screen, line and frame scanning means associated with said tube for deflecting said electron image over said image screen, and an electron-optical focussing system for focussing the secondary electrons emitted by said image screen to form an image on said picture screen.

5. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated inamplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, means incorporated in said tube for forming an electron image of said train on said image screen, line and frame scanning means associated with said tube for deflecting said electron image over said image screen, and a plurality of secondary electron multiplying electrodes in said tube and arranged between said surface and said frame scanning means.

6. Television receiving apparatus according to claim 1 wherein said light source is monochromatic. I

7. Television receiving apparatus according to claim 1 wherein said light source and said image screen are adapted to emit light of different spectral ranges, and said photo-electric surface is adapted to exhibit its maximum sensitivity in the spectral range of the light emitted by said light source.

8. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen. said surface and said screen lying in planes inclined at an angle to one another, a supersonic wave light modulating device,

a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, electronoptical means associated with said tube for bending the electron beams issuing from said surface sothat they fall on said screen at normal incidence, an electron-optical focussing system in said tube for fccussing said beams to form an electron image of said train on said screen, and deflecting means associated with said tube for deflecting said electron image over said screen to immobilize the picture points represented in said electron image with respect to said screen.

9. Television receiving apparatus according to claim 1 wherein the ratio of the Width to the length of said optical image is greater than the ratio of the width to the length of said electron includes an electron-optical fccussing system having a different optical power in the length direction of said images than in the direction at right an les thereto.

10. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, two supersonic wave li ht modulating devices, a light source for ill minatin said devices. means for applying picture'si nals in parallel to said devices to produce therein identical trains of moving supersonic waves modulated in amplitude in accordance with said si nals. optical means for forming optical ima es of said trains on said surface, said optical images lying in the same strai ht line, means incorporated in said tube for forming electron images r-f said trains tn said image screen, said electron images bein s parated in the direction of their length by an amount such that corresponding points in them are one picture line length apart, and deflecting means associated with said tube for deflecting said electron image over said screen to immobilze the picture p ints represented in said electron images with respect to said screen.

11. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitudein accordance with said signals, optical means for forming two identical images of said train on said.

surface, said optical images lying in the same straight line, means incorporated in said tube for forming electron images of said train on said image screen, said electron images being separated in the direction of their length by an 12. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic Waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, means incorporated in said tube for forming an electron image of said train on said image screen, line and frame scanning means associated with said tube for producing line and frame deflections of said electron image over said image screen, and means for produc ing a periodical relative increase of the excitation of said image screen during a portion of each line deflection.

13. Television receiving apparatus according to claim 12 wherein a saw-tooth voltage having a frequency equal to the line scanning frequency is superimposed on said picture currents and fed to said light modulating device.

14. Television receiving apparatus according to claim 12 wherein a saw-tooth voltage having a frequency equal to the line scanning frequency is fed to said electron image forming means.

15. Television receiving apparatus comprising a cathode ray tube having a photo-electric surface and an image screen, a supersonic wave light modulating device, a light source for illuminating said device, means for applying picture signals to said device to produce therein a train of moving supersonic waves modulated in amplitude in accordance with said signals, optical means for forming an optical image of said train on said surface, means incorporated in, said tube for forming an electron image of said train on said image screen, frame scanning means associated with said tube for producing frame deflections of said electron image over said image screen, line scanning means associated with said tube for producing line scanning deflections of said electron image over said image screen and to immobilize the wave images thereon and means for periodically applying a suppressing voltage to an electrode in said tube to suppress the formation of said electron image during a portion of each line scanning period. I

16. A method of television reception which and deflecting the electron beams which form I said image in such a sense andat sucha velccitythat the picture points represented in said electron image are immobilized with respect to said image'screen.

1 17. Television receiving apparatus comprising:

a comprises forming an optical image or: a train 3 i 01' moving supersonic waves existing in a liquid,

the amplitude of said waves being modulated in g accordance 'with' received picture signals, 'converting'said optical image into. a corresponding electron image on the surface or an ima escreen,

moving" su'pe'rscnivz: waves modulated in screen; deflectingmeans associated with said tube a' cathode ray tube having a'Dhoto-electrlc surj face and an image: screen, a supersonic wave light modulating device; a light source ior illuminating said device, means for applying pici'or deflecting said electron image over said screen 1 to counteract with respect to said screen the I a movement, due to the progress of thevsu-personic waves, of the picture points represented in said electron image; and further associated with said 1 tube electron image.

'ADOLPH HENRY ROSENTHAL.

deflecting -means I a I for applyinga frame 5 scanning movement to said 

